"The guy who owns a mansion wears them and the guy that cleans his swimming pool wears them, too," Fernando Tigre, the former president of Alpargatas SA, maker of the popular Brazilian flip-flop Havaianas, liked to say.

For millions of Third World buyers, flip-flops — aka thongs, zoris, slaps, flaps, beach walkers, etc. — are their first footwear. Often they're the first step in upward mobility toward closed-toe shoes, according to Sonja Bata, whose Toronto-based company, the Bata Shoe Organization, operates factories that make basic shoes in 26 countries.

The production of rubber-soled versions, which had begun in Kobe, Japan, in the 1930s, stoked Japan's recovery after World War II, technology historian Edward Tenner wrote in his book "Our Own Devices." Cutting and assembling them took so little capital, machinery and expertise — and they were in such demand — that many Japanese families and entrepreneurs got back on their feet making flip-flops. Mitsubishi, the Japanese conglomerate, bought out many of those businesses and became a big early exporter of flip-flops, according to Phillip Nutt, a Toronto-based shoe industry consultant.

Chasing lower wages, Japan's footwear companies moved production to Taiwan and Korea in the '60s and '70s and ultimately to China. Along the way, most transitioned into more profitable closed-toe shoes and then athletic shoes. At one stage or another in this still-continuing process, assembling flip-flops became a backyard occupation for "hundreds of thousands of families in Asia and Africa," according to Bata, who's been in the business since the 1940s.

Easy to make, easy to fit, simple and cheap, flip-flops have been virtually unchanged for more than 70 years, although not unchallenged within the sandal category. The U.S. Patent Office library catalogs sandals with toe guards and sandals that float. Sandals with air cushions and sandals with slots for insertable coolants. Even sandals with drain holes, laced uppers, and compartments you can put keys in. But the unpatented old-fashioned flip-flop rules.

Nutt, who's worked with flip-flops since the '60s, estimates total worldwide sales in the flip-flop category at 20 billion.

By comparison, sales of all athletic shoes, everything from cross-trainers and cleats to tennis and running shoes, total 17.8 billion, according to Mike May, spokesman for the Sporting Goods Manufacturers Association.

Donald Kalfin, the president of New York-based Sino-East International Ltd., a longtime Asian shoe marketer, said it's hard to lose on flip-flops because as a consumer product, they're the ultimate perennial: "If population grows, sales grow."

Archaeologists say sandals ancestral to the flip-flop are the world's second oldest footwear, after moccasins. Moccasins came first because they required only that feet be wrapped in animal skin. Sandals, which required a firmer sole, were made for millennia with wooden soles or of woven bamboo or straw. Rubber soles made them far more durable, and flip-flops spread throughout the Far East wherever there were rubber trees.

If today's flip-flops seem lighter in weight, you're right. Since the '60s, most have been made of a plastic called ethylene vinyl acetate. The sole is softer and lighter than a rubber flip-flop's, thanks largely to tiny air bubbles blown into the plastic. The newer versions also are better insulators against hot sand and more resistant to the sun's ultraviolet radiation than rubber ones.

Flip-flops reached Hawai'i by the late '30s and the U.S. Mainland by the '50s via West Coast surfers and Navy sailors, who preferred them when off duty.

"Off duty" is still the message that flip-flops give off, except for lifeguards. That's why Northwestern University's national champion women's lacrosse team caused such a flap last year when four of its members wore flip-flops to a White House event that honored them.

"They're just not for a formal occasion," shoe fashionista Meghan Cleary ruled.

The downside for flip-flops may be their indestructibility. Like many plastics, they're a solid-waste problem that tends to show up on beaches.